feat: plugin interface for key events via ya.which()

[sxyazi]

In async plugins, you can use ya.which() to prompt users with a set of available keys:

return {
  entry = function()
    local key = ya.which {
      cands = {
        { on = "a" },
        { on = "b", desc = "optional description" },
        { on = "<C-c>", desc = "key combination" },
        { on = { "d", "e" }, desc = "multiple keys" },
      },
      -- silent = true, -- If you don't want to show the UI of key indicator
    }

    ya.err(key)
  end,
}

When the user clicks a valid candidate, ya.which returns the 1-based index of that "cand"; otherwise, it returns nil, indicating that the user has canceled the key operation.

ya.which.mp4

If you want to use the key in a sync plugin, you can awaken the plugin in an async plugin by:

ya.manager_emit("plugin", { "your-sync-plugin", sync = "", args = tostring(key) })

The next target will allow direct calls of sync code within async plugins, making it much simpler. For now, please proceed with this approach.